The present invention relates to a semiconductor integrated circuit device, and particularly to a semiconductor integrated circuit made up of insulated gate-type field effect transistors (hereinafter referred to as MOSFET's) including a differential circuit type input circuit.
In a semiconductor integrated circuit device (hereinafter referred to as IC) with a differential input type digital signal input circuit, the level of a signal introduced from an external unit is compared with a reference potential or a reference voltage by the input circuit. The input threshold voltage of the input circuit of this type is determined not by the threshold voltage of the MOSFET but by the reference voltage. Therefore, the input circuit has an advantage in that the input threshold voltage can be suitably set.
In order to reduce the number of external terminals of the IC, the reference voltage should be produced within the IC.
To produce the reference voltage, a reference voltage generating circuit consisting of a voltage dividing circuit can be formed in the IC.
In this case, however, attention should be paid to the fact that undesirable variations are imparted to the reference voltage. For instance, variations in the power-supply voltage supplied to the reference voltage-generating circuit caused by the voltage variations developing in the power-supply wiring in the IC cannot be neglected. The reference voltage is changed by such variations in the power-supply voltage. Further, there exist undesired electric couplings between the wiring (reference voltage wiring) the reference voltage is applied to and the signal wirings in the IC that a variety of signals are applied to. Hence, the reference voltage wiring receives noise through these electric couplings.
The input threshold voltage of the input circuit undergoes changes due to the change in the reference voltage supplied to the input circuit. Consequently, the input circuit often operates erroneously even when the signals from the external unit are introduced at a suitable level.
In order to reduce the change in the reference voltage, it can be contrived to connect a capacitor between the reference voltage wiring and the grounded power-supply wiring (ground wiring) of the circuit in the IC in order to absorb changes in the voltage.
It was, however, found that the resistance or inductance of the ground wiring in the IC cannot be neglected, and that the change in the operation current in the IC causes changes in the voltage of the ground wiring to a degree that cannot be neglected. Therefore, even if the capacitor is connected to absorb variations in the voltage as mentioned above, it cannot sufficiently prevent undesirable operation of the input circuit.